Understanding Substring in Left Join Condition

When working with databases, especially when performing joins between different tables, it’s common to encounter situations where you need to manipulate data within the join condition. One such manipulation is extracting a substring from a string field using SQL functions like SUBSTR. In this article, we’ll delve into how to achieve this in a left join condition.

Background and Assumptions

To approach this problem, let’s first understand what’s happening under the hood. The question mentions two tables, xxlhoreca-bi.PriceSearch.XXL_PriceComparison (aliased as ps) and DataImport.CategoryUID (aliased as CategoryUID). It also references a table in the DataImport schema called DataImport.CategoryMappingWithLocalIDReporting_ID (aliased as category_mapping).

Given this setup, we’ll make some educated guesses to help us better understand what’s going on. Here are our assumptions:

• DataImport.CategoryMappingWithLocalID.Reporting_ID is indeed a numeric field in the original table.
• The goal is to extract categories from the reporting_id field within the first four characters.

Problem Analysis

The original query tries to use a substring function (SUBSTR) directly within the join condition. However, SUBSTR requires a string as input, but the referenced field appears to be numeric based on our assumption in Step 3. This leads us to believe that turning this dataset.table.field reference into a string by putting it in single quotes might not actually achieve what we want.

Solution Overview

Our solution involves two key steps:

1. Using the table alias in the join condition.
2. Converting the numeric field to a string using a casting function.

By combining these elements, we can successfully extract substrings from our target field while performing the left join.

Step-by-Step Solution

Using Table Aliases in Join Conditions

To leverage the table alias category_mapping effectively, we need to include it within our join condition. This is where our first key step comes into play.

JOIN `DataImport.CategoryMappingWithLocalID` AS category_mapping

This tells SQL to use the results from DataImport.CategoryMappingWithLocalID when performing the left join with CategoryUID.

Converting Numeric Fields to Strings

Now that we have our table alias in place, let’s focus on casting our numeric field (Reporting_ID) into a string. We’ll use SQL’s built-in CAST function for this purpose.

ON 
SAFE_CAST(SUBSTR(CAST(category_mapping.Reporting_ID AS STRING), 4) AS INT64) = CategoryUID.Category_ID

Here, we’re taking the numeric value from the reporting_id field, casting it into a string using CAST, and then extracting the desired substring. The INT64 cast ensures that any subsequent operations on this converted value are done in an integer context.

Putting It All Together

Once you’ve incorporated these two elements into your query, you should now be able to successfully extract substrings from your target field while performing a left join.

SELECT    
    IF (ps.shop = 'NL',TopCat.Parent_Title, CategoryUID.Parent_Title) as Parent_Title,
    IF (ps.shop = 'NL',TopCat.Sub_Title_1, CategoryUID.Sub_Title_1) as Sub_Title_1,
    IF (ps.shop = 'NL',TopCat.Sub_Title_2, CategoryUID.Sub_Title_2) as Sub_Title_2,
    ps.ean, ps.product_resource_id        
FROM `xxlhoreca-bi.PriceSearch.XXL_PriceComparison` ps
LEFT JOIN 
    `DataImport.CategoryMappingWithLocalID` AS category_mapping
ON 
SAFE_CAST(SUBSTR(CAST(category_mapping.Reporting_ID AS STRING), 4) AS INT64) = CategoryUID.Category_ID
LEFT JOIN 
    `DataImport.CategoryUID` CategoryUID
ON 
    SAFE_CAST(SUBSTR('DataImport.CategoryMappingWithLocalID.Reporting_ID', 4) AS INT64) = CategoryUID.Category_ID
GROUP BY 
    1, 2, 3, 4, 5

Conclusion

In this article, we explored how to successfully use substring functions within a left join condition. By making educated guesses and combining the table alias with string casting techniques, we were able to accomplish our goal.

Remember, when working with SQL databases, understanding your data structures and manipulating them using SQL functions like SUBSTR can be incredibly powerful tools in the right context.

Last modified on 2024-06-05